f you don’t register the servitude, you will have to prove (and the onus of proof here is on you) that subsequent owners actually knew of it when purchasing the property. 

That’s not going to be easy, and the dangers of relying on an unregistered right of use are illustrated in a case recently before the High Court.  The original owner of a farm had sold it subject to retaining a life-long right to remain in occupation of the farmhouse.  However when the farm was sold on, he was unable to prove that the subsequent buyer had had any knowledge of his rights.  The Court accordingly ordered his eviction from the farmhouse. 

And although the final owner had initially granted him a right to keep his cattle on the farm, the Court held that this right – being unregistered - could be terminated on reasonable notice, which had been given.  So his livestock had to go too.